The present invention relates to an interference reducing device which detects and reduces unmodulated carriers, narrow-band FM carriers, or their sweep carriers which get mixed as interference carriers into the communication band in a TDMA (Time Division Multiple Access) digital satellite communication system.
With the development of satellite communication systems, the number of earth stations which access the same satellite is on the increase. Occasionally the situation arises where un-modulated carriers, narrow-band FM carriers, or their sweep carriers are emitted as unwanted carriers owing to a fault or operation mistake of earth station equipment. In the TDMA system each station transmits, every fixed period of time called a TDMA frame, a burst-like TDMA signal of a fixed carrier frequency within a certain time slot allotted to the station. To establish burst synchronization for high precision control of the burst transmission timing of respective stations to ensure that they send their bursts at different timing, a reference station sends a reference burst for each TDMA frame. The reference burst is used for system control as well as for maintaining the synchronization of the TDMA system. On the other hand, a traffic station for transmitting and receiving information sends a traffic burst. The application of the above-mentioned unwanted carriers as interference carriers to the TDMA system not only degrades the signal quality but also may sometimes hinder the system control by the reference burst and synchronization of the traffic burst, making communication impossible throughout the system.
In the time division multiple access system (the TDMA system) introduced in the digital satellite communication, each station transmits a burst-like TDMA signal of a fixed carrier frequency within a certain time slot allotted to the station. To establish burst synchronization for high precision control of the burst transmission timing of respective stations so that they send their bursts at different timing, a synchronization burst is inserted for each TDMA frame. The synchronization burst (a reference burst) sent from a reference station is employed for system control as well as for maintaining the synchronization of the TDMA system. An interference carrier, if mixed in such a communication system, will degrade the signal quality and, further, may sometimes destroy the burst synchronization and the system control by the reference burst, leading to a failure in communication.
For detecting an interference carrier mixed in the input signal band, there has been proposed a method which obtains the interference carrier frequency from the output frequency of a voltage-controlled oscillator which is synchronized with the narrow-band interference carrier by means of a phase lock loop, or a method in which the input signal band is divided by a filter multiplexer composed of a plurality of narrow band-pass filters of different center frequencies and a signal component in each divided band is detected, converted into a digital value and processed by a CPU for estimating the interference wave carrier frequency. The interference detector employing the phase lock loop is simple in circuit construction and permits easy detection and tracking of an unmodulated carrier or interference carrier which continuously sweeps in the received signal band, but in the case where the interference carrier mixed in the input signal is discontinuous or intermittent, synchronization of the phase lock loop cannot be maintained. Further, when the level of the interference carrier becomes low, the synchronized state of the phase lock loop becomes unstable, making it difficult to decide the presence or absence of the interference carrier. On the other hand, the filter multiplexer system is capable of detecting an interference carrier, even if it is discontinuous or intermittent, but in a case of detecting an interference carrier of power lower than that of the input signal, it is necessary to use a filter multiplexer of enhanced detecting sensitivity by dividing a desired transmission band with a number of narrow-band filters, a number of A/D converters for converting the detected outputs of the multiplexer into digital values and a CPU of high-speed operation. This inevitably makes the interference detector bulky and expensive.